This present invention relates to white sheet and boards for the printing and display of graphics in commercial and retail signage.
Printing technology has made major advancements over the years in both software and hardware to allow for a safer more consistent way to reproduce graphics in various printing platforms. Heavy metals have been taken out of the inks and safer solvents along with water based inks and ultra violet curing has changed the press room. Flatbed UV printers have been added to sheet and roll fed presses. These new digital flat presses can print on large rigid sheets with dimensions of 4′×8′×1″ and larger. Major applications for these large flat lightweight signs are for interior retail graphics. These signs can be seen at many large retailers such as anchor stores in shopping malls, supermarkets, and the like. There are many different store locations for graphics. Some locations are high in the air and require large rigid self-supporting panels while other locations may require thin flexible materials to slide into a frame. One thing that is critical is a brilliant white colored substrate to print on. For the most part most of the inks used in the printing process have a degree of transparency and the ability of the color to pop is due to the white paper reflecting light back through the ink. Not only will the white substrate give the ink some of its brightness but it will also influence the color of the ink unless the colors are dark and opaque. Small changes in the substrate's white shade and brightness will sometimes have dramatic changes on the colored inks appearance both from a visual perspective and spectral data curve.
The preferred white color for a particular application or customer may vary from warm to cool, however once a color of white is selected it is critical that the white be consistent for that job or customer or application. If the white varies then the color and appearance of the printed material will vary and not look the same. When company logos and colors are printed they must look the same time and time again and on specification.
Companies will also use different types of print substrates depending on the application. Some graphics are long term signage as others are short term and disposable. Plastic (e.g., polystyrene) may work for more permanent signage where clay coated paper faced composite boards may be for short term use. Paper faced and plastic faced rigid boards are the main substrates used in the aforementioned applications. The core of these laminated boards may be cellular foam or paper honeycomb. From a manufacturing perspective it is not easy to have different types of materials (paper, plastic) match in white color. However it is very important to be consistent color from printing both and having them in the same location.
One can appreciate that the white color in a plastic sheet is accomplished by incorporating a white pigment (i.e.,TiO2) into the resin during the extrusion process for a homogeneous dispersed color. For the paper sheet version of the paper faced board the paper fibers are bleached to a white color and white pigments and additives may be dispersed in the wet end of the paper machine into the fiber matrix and in many cases a white pigmented clay based coating containing TiO2, calcium carbonate, and optical brightener is coated on one or two sides of the sheet.
With some effort one can come close to having the same shade of white color on both the plastic and paper sheet as long as one uses a specific light source to view the two (paper and plastic) and match the two during manufacturing using that one light source.
It is well known in the industry and the retail environment that the color of an item can change dramatically when taken outside or into different lighting environments (i.e. daylight, incandescent, fluorescent, halogen, mercury vapor-convention hall). The real challenge as outlined above is to have the paper and plastic sheet matched in manufacturing for viewing under perhaps a standard D50 proofing illuminant, also match under all the different light sources mentioned above. This task has been very challenging for a number of reasons.
Optical brighteners (hereinafter, OB) are also known as florescent whitening agents. These additives are put into almost all white papers and many white plastic consumer products to make them appear whiter and brighter. These optical brighteners absorb invisible ultraviolet light in the 300-400 nm range and make it re-emit or fluoresce in the blue region of the visible spectrum peaking at 435 nm. This effect will compensate for a yellow tint of many types of white paper pulps that have been bleached and plastic resins with a yellowish cast present in the base resin or visible due to high heat during post processing or with the addition of additives as UV absorbers light stabilizers, processing aids, antioxidants, lubricants, heat stabilizers, surface modifiers, and fillers. It would be difficult to find a white paper from a paper mill that did not have optical brighteners; if it had no OBs it would have a yellow tint and not be suitable for printing display graphics and most other printing applications.
Papers and plastics with optical brighteners will appear to change in color based on the amount of UV light emitted by the viewing light source and the amount of OBs in the paper and plastic and their ability to fluoresce based on the chemistries of the products (i.e. pigments, additives) that may compete with the OB to quench their effect.
Optical brighteners are used in the paper and plastic industries to mask the yellow nature of many bleached paper fibers and thermoplastic resins used in the household article and packaging industries. Some of these applications are in clear resins and colorless items while others are white pigmented articles. The differences in concentration levels of optical brightener between the clear OB resin product and the white pigmented product can be 50 plus times greater in white than clear. Since the cost of the optical brighteners can be 20 times or more the cost of the base plastic resins or paper pulp attention must be given to the method of use and affordability.
The appearance of a typical optical brightener in its raw manufactured state is a yellow crystalline powder. One must not overdo the level of use and thereby defeat the reason for its use in the first place.
White plastic sheet in the range of 0.010-0.080 inch (10-80 mils) would be typical for the aforementioned plastic to be laminated to the two sided foam board or used as a single flexible plastic print sheet. These sheet materials must exhibit strong opacity so graphics printed on both sides of the sheet would not show through if displayed in a retail environment. This requirement would dictate a high loading of pigment with TiO2. Since TiO2 has a slight yellow tint and the preferred resin for the rigid board and flexible sheet application is polystyrene with its own yellow cast one can see that the optical brightener, the pigment, and the resin all push the extruded plastic sheet to a slightly yellow shade. One can compensate by adding a small amount of blue or violet colorant into the resin to mask the yellow. This tends to cut back on the brightness and makes the white a bit dirty.
The greater the amount of TiO2 one loads into the resin the less positive effect OBs will have of masking a yellow shade in the sheet. This is due to the interference and masking effect of the TiO2 not allowing the OB to be absorbed and fluoresce. The more OB added to the sheet to compensate for the high loading of TiO2, the more a yellow tint appears in lighting environments with minimal UV light present.
An additional problem occurs when a plastic print sheet is displayed in sunlight, or for extended periods in artificial light. The UV light component can not only provide the advantage of interacting with the OB to enhance brightness, but also raises the disadvantage of reacting with the resin polymers to produce an unwanted yellowing that is quite visible to the observer.